Defining the Isotropic Mass Value
You specify a mass magnitude, which is associated with the three translational degrees of freedom at the node of the element. Specify mass, not weight. You must associate this mass with a region of your model.
Defining the Mass Matrix Explicitly in Abaqus/Standard
You can define a general mass matrix explicitly in Abaqus/Standard if the introduction of individual terms on and off the diagonal of the mass matrix is desired. See User-Defined Elements for details.